Generally, an image forming apparatus such as a copier, printer, and/or facsimile includes a fixing unit to fix a toner image on a recording medium. Such recording mediums include a transfer sheet and an over head projector (OHP) sheet. The fixing unit fixes the toner image on the recording medium by applying heat to the toner image through a fixing member heated by a heating source.
For example, the fixing unit may employ a heat roll method or a belt fixing method. In the case of the heat roll method, a heating source such as a halogen heater heats a fixing roller, which is pressed by a pressure roller. The fixing roller and the pressure roller form a nip portion therebetween. In this way, a toner image can be fixed to a recording medium by applying heat and pressure to the toner image on the recording medium when the recording medium passes through the nip portion.
Recently, environmental concerns have prompted studies calling for the reduction of energy consumption in image forming devices. To reduce energy consumption of a fixing unit of an image forming apparatus, the consumption of the overall device needs to be considered.
To reduce energy consumption of the fixing unit of the image forming apparatus in a stand-by mode, the fixing roller can be maintained at a temperature, which is slightly lower than a fixing temperature. With such a method, when a user wants to start an image forming mode, the fixing roller can be heated to a fixing temperature in a shorter period of time. This method avoids longer waiting times before an image forming process is actually conducted. Accordingly, some electric power is consumed to maintain a temperature of the fixing unit when the image forming apparatus is in the stand-by mode.
Yet, it is preferable to reduce energy consumption during the stand-by mode of the image forming apparatus, and more preferable to reduce energy supply to zero during the stand-by mode of the image forming apparatus.
If energy supply to the fixing unit is set to zero during the stand-by mode, the fixing roller, which is mainly composed of metal having a larger heat capacity such as iron and aluminum, needs a relatively longer waiting time to be heated to a fixing temperature (e.g., 180 Celsius degree) when a user instructs an image forming mode. Such waiting time may be several minutes, for example. In such a case, a user is inconvenienced by such a long waiting period.
In order to shorten the heating time of the fixing unit, a fixing member can be heated at a temperature, which is higher than a fixing temperature before rotating the fixing member and a pressure member. A heating time of a fixing unit can also be shortened by increasing the power supplied to the fixing unit per unit time. For example, some image forming apparatuses have a configuration that can be connected to a power source having a higher voltage such as 200-voltage to attain a higher printing speed.
However, using a higher voltage power source may not be practicable in some geographical areas as a generally used commercial power source in such areas may utilize a lower voltage such as 100-voltage (with 15 amperes). A high voltage power source can be used in a lower voltage area such as Japan, but a special electrical arrangement is required to use the power source of higher voltage, thereby it is not practicable to use the power source of higher voltage to shorten the rise-up time of an image forming apparatus.